The prior art recognizes the utilization of polymers that can be formed into films, membranes, coating, woven and non-woven layers and similar structures, which structures have blood compatibility, hydrolytic stability and gas permeability. Numerous types of gas permeable membranes for use in gas separation devices, blood-gas exchange devices and methods for preparation of these membranes also exists in the prior art.
Major prior art references related to the use of such polymers in association with blood analysis, sampling, gas exchanges and similar processes are as follows:
U.S. patent to R. J. Peterson entitled Blood Compatible Polymers for Blood Oxygenation Devices, U.S. Pat. No. 4,008,047, issued Feb. 15, 1977;
U.S. patent to Theodor Kolobow, entitled Blood Compatible-Gas Permeable Laminated Carbon Containing Silicone Rubber Membrane, U.S. Pat. No. 4,093,515, issued June 6, 1978;
U.S. patent, assigned to General Electric Co., entitled Organopolysiloxane-Polycarbonate Block Copolymers, U.S. Pat. No. 3,189,662; and,
A publication "Interactions of Materials with Blood" entitled Blood Compatible Synthetic Polymers by S. D. Bruck published in 1974.
This latter publication provides a summary of the problems of bringing organic and inorganic foreign substances into contact with the blood of a living animal. Some of these problems include the loss of platelet preservation and activity and damage to proteins and cellular material in the blood.
The applicant has found that the membranes provided by these prior art membranes have been subject to various problems. Such problems include, but are not limited to the following: hydrolytic instability in the case of fluorinated ethylcellulose at pH levels of 9 and polyethylene-vinyl alcohol copolymers at pH levels of 7; inability to autoclave or sterilize satisfactorily in the case of organo-polysiloxane-polycarbonate block copolymers; carbon dioxide removal limitations in the cases of silicone rubber fabric reinforced silica filled membranes, fluorinated ethyl-cellulose organopolysiloxane-polycarbonates, polyfluoro-siloxane and laminated carbon containing silicon rubber membranes; relatively low gas permeabilities requiring larger than necessary surface areas for blood contact and gas exchange with all of the above; and hematological damage due to surface irregularities such as protruding silica crystals on the exposed surface of silicone rubber membranes and fluorosilicone silica reinforced membranes; and poor physical strength properties and high costs in the cases of polyalkylsulfones, polyfluoro-siloxanes and the rest of the above.
Similarly, with other materials that have been used in the prior art, various problems exist. Gas transfer inadequacies and blood damaging characteristics occur with the use of uncoated microporous materials such as teflon, polypropylene and silicone hydrofugated polynosic fabric. The damaging characteristics are not due to poor gas transfer rates, but rather to a carbon dioxide flux decline over time due to water vapor condensation on the gas side micropores, and alternatively, the flooding of blood side micropores. This excessive water vapor loss is implicated in causing hyperosmolar states and, hypernatremia. This flux and condensation with uncoated microporous membranes requires the use of a heated and humidified gas supply in order to minimize these effects but the drawbacks limit the useful life of microporous membrane oxygenators to 6 to 8 hours and these are still implicated in causing deposition of blood-derived material.
It has now been found that the gas permeability, blood compatibility, hydrophobicity, ease of manufacture, mechanical strength, handling properties, hydrolytic stability and other desired properties for polymers used for gas permeable membranes or in the mentioned biological environs can be effectively combined in a composite membrane consisting of two phase siloxane-polyarylene polyether block copolymers cast upon and onto a microporous substrate. The use of ultrathin block copolymer membranes cast upon a microporous support material has not been recognized as having excellent utility in fields requiring blood compatibility or gas permeability.